A tablet printing apparatus is used to print identification information such as letters or characters (alphabet, kana character, number, etc.) and marks (symbol, figure, etc.) on the surface of a tablet for identifying the tablet. As such tablet printing apparatuses, those that perform printing on tablets in a noncontact manner have been developed due to the ease of changing identification information, high print quality, and the like. An inkjet tablet printing apparatus is configured to eject ink (for example, edible ink) toward tablets while conveying them by a conveyor belt, thereby printing identification information on the surfaces of the tablets. The legibility of the identification information printed on the surface of each tablet is influenced by the density of ink dots formed thereon by the ink ejection from the inkjet head, namely resolution.
The inkjet head of the tablet printing apparatus includes an array of nozzles that are arranged perpendicularly to the tablet conveying direction (hereinafter also simply referred to as “conveying direction”) in the horizontal plane. In tablet printing by the inkjet head, the resolution in the conveying direction according to the tablet conveying speed is determined by the control of timing of the ink ejection from the nozzles of the inkjet head according to the tablet conveying speed. On the other hand, the resolution in a direction perpendicular to the conveying direction is determined by the nozzle pitch in the nozzle array direction. The minimum value of the nozzle pitch of the nozzle array is determined by a limit in processing. Therefore, in order to increase the resolution in the direction perpendicular to the conveying direction, a method of arranging nozzle rows is adopted. Specifically, a plurality of nozzle arrays is used and the nozzles in the nozzle arrays are shifted to be placed in a staggered arrangement (zigzag arrangement) with respect to the conveying direction. For example, nozzles in the second array are each located to face the middle of the nozzle pitch of the nozzle array in the first array.
When printing is performed with the use of a plurality of nozzle arrays as described above, the position and posture (for example, inclination, orientation, etc.) of each tablet may change due to the vibration of the conveyor belt or the like while the tablet moves between the nozzle arrays. In particular, the longer the distance between the nozzle arrays used for printing, the more the position and posture of the tablet are likely to change during printing. A change in the position or posture of the tablet during printing causes a shift in the print position on the tablet where a print is applied by the inkjet head. Accordingly, identification information printed on the tablet is not clear, and the legibility thereof is reduced. As compared to ordinary printing, legibility is important for printing of identification information on tablets. Low legibility can lead to, for example, the misreading of information (such as the type and amount (dose) of medicine) printed on tablets, resulting in misuse of the medicine.